Aloha my friends!
If you’ve ever had a knee injury, or even just knee pain… you know how difficult it can be to work your quads in that state. Often just trying makes the pain or injury worse.
By far the most common quad-specific lift people use are leg extensions — usually in a sitting position. But if you have a knee injury, you want to AVOID leg extensions (unless you’re doing them with extremely light weight combined with BFR). Leg extensions put an enormous amount of strain on the knee, and doing them too often and/or too heavy is actually the root cause of many lifting-related knee injuries.
Perhaps the single best exercise you can do to target the quads that will not only allow you to effectively build them even with existing knee pain, but will also actually help to rehab the knee injury simultaneously, are backwards sled drags!
For one, it’s what’s called a Closed Kinetic Chain quad exercise… which essentially means that your foot is fixed and supported (in this case by the ground) while the quads are engaged. Closed Kinetic Chain movements in general put much less stress on the knee joint than Open Kinetic Chain exercises (like leg extensions) do.
On top of that, backwards drags limit the range of motion in the knee, never bending it more than 45°. And you have a significant amount of control over that range of motion. If you wanted, you could just take smaller steps and limit the amount of knee flexion to just 15° or less.
Now, you may be thinking that you could do the same with a squat, and simply limit it to a shallow range of motion. While that is true, squats are far less quad-specific than the backwards drag is, so you’d have to add significantly more total weight to the lift in order to get the same amount of load on the quads themselves, increasing the likelihood of aggravating the knee injury. Additionally, your center of gravity is much farther forward / over your knees with a squat, and the angle of resistance is directly down (gravity) which also adds strain to the joint.
With the backwards drag, your center of gravity is much farther back behind your knees, and the angle of resistance is more horizontal – both of which serve to decrease the strain on the knee, which simultaneously increasing the tension / focus on the quads! And working your quads in a way that protects the knee like this will serve to help heal & strengthen the knee – in part by increasing blood perfusion to the soft tissue around it.
If you don’t have access to a sled like the one above, you can get one here (this is the exact one I bought and use myself) or use the cable-based alternative I demonstrate in the GIF below:
Application: If you want to build your quads but have knee pain / injury (or want to PREVENT it), make backwards drags an integral part of your routine!!
Relevant Previous FTF Tips (for All-Access Members):
1) Week 81 | How to Work the Rectus Femoris Quad Muscle! (Sitting Leg Extensions DON’T)
2) Week 41 | Exercise Order Optimization – Hamstrings & Quads
Relevant Research Article(s):
1) Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises
Relevant Dr. Gains YouTube Video(s):
1) Knee Pain While Squatting? These Two Exercises Will SAVE You!!
2) 12 Outer Quad Regional Hypertrophy / Vastus Lateralis / Quad Sweep Techniques! (Science-Based RHT)
Relevant Dr. Gains Workout Program(s):
1) Hypertrophy Series Total Leg Programs (Beginner & Advanced Versions)
Dr. Gains 
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